In general, the surface sizing agent can be roughly classified into solution type and dispersion type. It can be said that the solution type is relatively susceptible to applied environment, particularly water (pH, hardness and the like). On the other hand, the dispersion type has the following drawbacks that when an anionic emulsifier is used as a dispersant, precipitation occurs by calcium ions and the like in water. This may induce deterioration of the performance as the surface sizing agent, and also cause the problems of machine contamination and bubbling. Therefore, it has conventionally been known to use a graft polymer using polysaccharide as a stabilizer, dispersibility might be insufficient depending on the type and the amount of a monomer used. In this case, dispersion stability can be improved by using jointly, for example, a surfactant having high ionic strength, whereas it becomes difficult to fix onto the surfaces of paper products. This may induce deterioration of sizing performance as the purpose of the surface sizing agent, and also increase bubbling. On the other hand, when a graft polymer of polysaccharide having low ionicity is used as a stabilizer, excellent hard water stability can be attained, whereas the interaction and fixing action with various types of ions existing in the paper become weak, thus making it difficult to attain high sizing performance.
Specifically, the following surface sizing agents have been heretofore reported.
That is, Patent Document 1 has proposed a sizing agent composed of a fine aqueous dispersion of a copolymer which is obtained by emulsion polymerization of 10 to 56 parts by weight of a monomer mixture consisting of 20 to 65% by weight of acrylonitrile, 80 to 35% by weight of acrylic ester of C3-C8 alcohol, and 0 to 10% by weight of other ethylene-unsaturated monomer in 100 parts by weight of an aqueous solution containing 15 to 25% by weight of a degraded starch having a specific viscosity, in the presence of a peroxide group-containing initiator. However, this sizing agent has insufficient dispersion stability and requires improvement.
Patent Document 2 has proposed a sizing agent which is obtained by emulsion polymerization of 40 to 140 parts of a monomer mixture consisting of 20 to 65% of acrylonitrile, 80 to 35% of acrylic ester of monovalent saturated C3-C8 alcohol and 0 to 10% of other ethylene-unsaturated copolymerizable monomer in 100 parts of an aqueous solution containing 2.7 to 15 parts of a degraded starch having a specific viscosity. However, this sizing agent also has poor dispersion stability and requires improvement.
Patent Document 3 has proposed a dispersion liquid of a copolymer containing a starch degradation product having a weight average molecular weight of 2500 to 25000, which is manufactured by radical polymerization of an unsaturated monomer, followed by hydrolysis in water phase. However, this dispersion liquid has the drawback that the molecular weight of the starch degradation product used as a stabilizer is relatively low, and a large amount of hydrophilic portions are liable to generate by the hydrolysis, thereby being capable of inducing deterioration of sizing effect.
Patent Document 4 has proposed, as a surface sizing agent, an aqueous emulsion polymer obtained by addition polymerization of a monomer in the presence of a starch degraded product and a polymer emulsifier such as sulfo-containing maleic anhydride copolymers or sulfo-containing oligourethanes having a branched molecular structure of terpolymers. However, this surface sizing agent uses jointly the polymer emulsifier, causing concern about foaming property compared with the case of using the starch degraded product alone.
Patent Document 5 has proposed a dispersion liquid of an aqueous polymer composed basically of a copolymer of styrene and (meth)acrylic ester, as a polymer dispersion liquid for paper sizing. However, this polymer dispersion liquid has a high content of the (meth)acrylic ester in the polymer, failing to impart sufficient sizing effect to the paper requiring strong water repellency.
Patent Document 6 has proposed a sizing agent whose water repellency imparting effect is improved by increasing the content of styrene and decreasing the content of (meth)acrylic ester in the polymer. However, this sizing agent contains neither ethylene-based unsaturated carboxylic acid nor sulfonic acid in the entire solid composition of a polymer and a dispersant. It is therefore difficult to say that sufficient effect can be expected in both sizing performance and dispersion stability.
Thus, all these related art surface sizing agents have certain problems, and hence a surface sizing agent is desired which is capable of exhibiting sufficiently satisfactory performance under any environment, irrespective of water characteristics (hardness, pH and the like) and the type of paper.
Patent Document 1: Japanese Unexamined Patent Publication No. 63-50596
Patent Document 2: Japanese Unexamined Patent Publication No. 63-203895
Patent Document 3: Japanese Unexamined Patent Publication No. 5-209024
Patent Document 4: Japanese Unexamined Patent Publication No. 11-240906
Patent Document 5: Japanese Unexamined Patent Publication No. 2002-504563
Patent Document 6: Japanese Unexamined Patent Publication No. 2002-527643